Vacuum pumps for creating or maintaining vacuum include various types, in which a dry screw pump can be mentioned as those providing relatively low vacuum degree. This comprises a pump main body having a suction port and a discharge port, a pair of rotors disposed in the pump main body and formed with left hand screws and right hand screws, respectively, and a rotor shaft rotationally held by means of rolling bearings in the pump main body, in which the pair of rotors rotate in synchronization in a state not in contact with each other, thereby sucking a gas in a vessel connected with the suction port into the pump main body and discharging the same from the discharge port to evacuate the inside of the vessel.
The two rotor shafts are rotationally supported by rolling bearings and usual bearing steels (SUJ2) are often used for the rolling bearings. Since high contact stresses are exerted repeatedly on inner and outer rings and rolling elements of the rolling bearing, it is necessary to harden the inner and outer rings and the rolling elements for extending the rolling contact fatigue life. Accordingly, after forming the inner and the outer rings and the rolling elements of the rolling bearing with the bearing steels (SUJ2), quenching and tempering are applied.
Further, as lubricants for bearings used in reduced pressure (including vacuum) atmosphere such as in vacuum pump apparatus, solid lubricants are sometimes used for preventing contamination of the reduced pressure atmosphere and the use of fluid lubricants such as lubricating oil has been increased in order to improve the life and the reliability of the bearings. As typical examples, highly corrosion resistance and less evaporative fluoro-lubricating oil are used. Particularly, in high vacuum pumps used at a high speed rotation and in a high temperature circumstance depending on the case, fluoro-lubricating oil with higher reliability have been used more frequently. Heretofore, in the rolling bearings used in such a reduced pressure atmosphere including vacuum, only the lubricant is changed to the fluoro-lubricating oil but the accuracy of the bearings per se is left as it is in the usual specification (for instance, at about JIS B 1514 for the accuracy).
On the other hand, fluoro-lubricating oil or fluoro-grease which is excellent in heat resistance, less evaporative and chemically stable is used also in a high temperature circumstance exceeding 200° C. such as in heat rolls for fixing portions of business machines. Such fluoro-lubricating oil or fluoro-grease can include, for example, lubricating oil or greases containing fluoro-containing polymers such as perfluoropolyether (PFPE) oil or fluoro-grease using the PFPE oil as a base oil. Under the usual working circumstances, there has been no problem for the lubrication with the lubricating oil or greases containing such fluoro-containing polymers.
However, business machines typically represented by copying machines (PPC), laser beam printers (LBP), facsimile (FAX) or composite machines thereof have tended to be adapted for more compact structure, energy saving, recycling performance and higher speed in recent years and, accordingly, it has been demanded for longer torque life with a smaller size and under severer conditions (high speed and heavy load). Further, also in the field of the vacuum pump, rolling bearings capable of operating for a long time at higher temperature and under higher speed have been demanded in order to increase the capacity and make the structure compact. Further, also in the field for manufacturing semiconductors and liquid crystal panels, operation for long time under large load and high speed is required for the rolling bearings to be used along with the enlargement for the size and high speed transportation of substrates.
Aside from them, as general materials for cages that hold rolling elements in an even distribution between an outer ring raceway and an inner ring raceway, SPCC material, HB25CL material and PA66 resin material have been used. For the rolling bearings required for corrosion resistance such as support bearings in the vacuum pumps, cages for use in rolling bearings formed into a circular shape with a advanced resin material which is poor in the ductility but has corrosion resistance is used. In the cage for use in the rolling bearing, a plurality of pockets each receiving a rolling element from an opening and holding the same rotatably are disposed each at a predetermined distance in the circumferential direction.
Referring to FIG. 24, size F for the opening of each pocket 30 is set to a value of 85 to 93% for the rolling element diameter G. Accordingly, each rolling element 31 is assembled from the opening 33 into each pocket 30 while circumferentially expanding fingers 32 of each pocket 30. This can make easy assembling of the rolling element 31 to each pocket 30 and less drop off of the rolling element 31 assembled in each pocket 30 compatible.